We thoroughly check everything from parts to assembly groups for particle residue, as we now know that cleanliness is a critical factor for reliability and service life. The components are washed and cleaned several times to remove clinging particles. To do so, tools such as lances are used to clean bores from the inside, or correspondingly high pressures are used during washing. After the cleaning, we examine the particles according to their size and distribution using the microscopic analysis method. The results are not only important for verifying our high quality standards, some of which are even more stringent than the official directives. They also help us to identify the cause and origin of the contamination.
What effect does particle residue clinging to components have?
Our components have narrow tolerances, high-precision fits and small microscopic openings. Tiny particle residues in injectors, pumps and control units can cause malfunctions with serious consequences up to and including failure of the entire fuel-injection system. Our injectors inject the fuel into the engine through bores that are almost unimaginably tiny. As you can surely understand, particles equal to or larger than a certain size at these places cause problems. The same holds true for the control system of the injectors – an interaction of different pressures which, in turn, are controlled via tiny bore holes. If a shaving gets stuck in a hole, the function of the fuel-injection system is impaired.
Optical inspection is the preferred technique for routine. It provides reliable results quickly, while still being cost-effective. We are not interested in whether our parts are "clinically" clean. The questions we have to answer are whether particles are present, how large they are and whether they can cause malfunctions. We classify the particles according to their size and assign them to functionally relevant classes. Depending on which class they belong to, corresponding scenarios exist from which, in turn, we can derive consequences for the manufacturing process. As a rule, very small particles tend to be uncritical. We record them in order to be able to detect any changes in the manufacturing processes at an early stage. In special cases, we use expensive, time-consuming scanning electron microscopic examinations – for example, if we want to study the exact composition of the particles.
We study parts and assembly groups at regular intervals according to our inspection plans. We study fully assembled injectors as well as parts. The test frequencies themselves are affected by experimental values. If a part suddenly shows elevated values, the test frequency is increased and corrective actions are introduced simultaneously.
We are very satisfied with the Leica solution for cleanliness analysis. We are particularly pleased with the self-explanatory operation. Leica Microsystems continuously develops the software and integrates things that make it easier to use. The different password-protected user levels – user, supervisor and service – make the system highly secure in everyday use. In the future, the industry wishes for a fully automatic solution for counting and classifying particles in order to increase the number of test parts per time unit and allow real-time, on-site inspections. For the time being, however, I do not see any way to implement these ideas cost-effectively.